Avocado tree named ‘Merensky 2’

ABSTRACT

A new and distinct variety of avocado tree having many characteristics similar to those of ‘Duke 7’ that is characterized by strong resistance to  Phytophthora cinnamoni,  and which bears more fruits than the ‘Duke 7’ variety when ‘Hass’ is grafted thereon.

Botanical classification: Persea americana Mill.

BACKGROUND OF THE INVENTION

The present invention generally relates to avocado trees. Moreparticularly, the present invention relates to a new and distinctvariety of avocado tree, Persea americana, with significantly improvedresistance to root rot.

A tree or seedling rootstock was discovered in an avocado orchardheavily infested with root rot, caused by Phytophthora cinnamoni atWestfalia Estate, South Africa. Most trees in that orchard died of rootrot, however, the tree of the present invention was one of the fewsurvivors and looked very healthy.

In the Westfalia nursery, an asexual reproduction of the tree rootstockwas made by taking a bud bearing stick from a rootstock shoot andgrafting onto a nurse seedling in the nursery. The grafted wood wassubsequently rooted following standard procedures for producing clonalavocado trees. The trees were grafted with the scion ‘Hass’ in thenursery and planted in field trials in which the tree always showed thesame superior resistance to root rot as the original seedling. Theclones or propagules of the tree have been found to be identical to theoriginal seedling in all distinguishing characteristics.

A Simple Sequence Repeat (SSR) DNA fingerprinting technique wasperformed on avocado leaves of various varieties, including the‘Merensky 2’. Three samples from the same cultivar were used. Aside fromleaves of the ‘Merensky 2’, three other rootstock selections from theWestfalia nursery, ‘Edranol’, ‘Ettinger’, ‘Fuerte’, and ‘Duke 7’ wereused. Total genomic DNA was isolated from the leaf samples using 2% CTABand chloroform:isoamylalcohol (24:1). The SSR primers were: A1E11,A4F08, A7G04, of which the A7G04 primer was found not useful and theA1E11 primer most useful in clearly distinguishing the cultivars, excepttwo of the rootstocks from the Westfalia nursery. No variation could befound between the different samples from the same cultivar. It wasconcluded that two of the cultivars from the Westfalia nursery wereidentical. However, there were differences found between the ‘Merensky2’ and the other varieties, and very clear differences found between the‘Merensky 2’ and the ‘Duke 7’ variety.

In California, the ‘Merensky 2’ avocado trees have been planted andgrown at the “University of California South Coast Field Station”, inIrvine, Calif.

SUMMARY OF THE INVENTION

The invention relates to a new and distinct variety of an avocado treehaving many characteristics similar to that of ‘Duke 7’. The inventionis characterized by superior resistance to root rot, such as that causedby Phytophthora cinnamoni, as compared to ‘Duke 7’. The fruit of thepresent invention has been found to be slightly larger than that of‘Duke 7’, and also has a better taste than that of ‘Duke 7’.

An additional report characteristic of ‘Merensky 2’ is its affect on thebearing of the ‘Hass’ avocado. ‘Hass’ grafted onto the instant cultivarbears more fruits than ‘Hass’ grown on ‘Duke 7’.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying photographs show the tree and fruit of the presentinvention. In such photographs:

FIG. 1 shows a ‘Merensky 2’ variety in South Africa;

FIG. 2 shows a mature ‘Merensky 2’ in California;

FIG. 3 shows flush from a ‘Merensky 2’;

FIG. 4 shows flush from ‘Duke 7’;

FIG. 5 shows a comparison of mature leaves of the ‘Duke 7’ (left) and‘Merensky 2’ (right);

FIG. 6 shows a comparison of succulent stems of ‘Duke 7’ (left) and‘Merensky 2’ (right);

FIG. 7 shows the ‘Merensky 2’ in bloom;

FIG. 8 shows the ‘Duke 7’ variety in bloom;

FIG. 9 shows an exterior view of a mature fruit of the ‘Merensky 2’variety; and

FIG. 10 shows views of the fruit flesh and stone of the ‘Merensky 2’.

BOTANICAL DESCRIPTION OF THE PLANT

In those instances where precise color assessment can be made,references are to The Royal Horticultural Society (R.H.S.) Colour Chart.In other instances, generally, color terms are used in accordance withan ordinary dictionary significance. The instant cultivar ‘Merensky 2’is described as a plant as a whole in the following description, withthe exception as an rootstock for a specific scion when reference ismade to root rot resistance and yields. The following description istaken from a four year old cultivar located at Groenkloof Block 2,Westfalia Estate, South Africa.

Cultivar name: ‘Merensky 2’.

Botanical name: Persea americana.

Form: Tree.

Cultural conditions: Westfalia Estate, Groenkloof section, is situatedin north-eastern South Africa (latitude 23.45 S, longitude 30.05 E,altitude 750 m above sea level). The soil type is a fine-loamy, mixedpaleuduit (USDA), 1975. Soil Taxonomy, Soil Conservation Service,Agriculture Handbook No. 436, Washington) with a clay content of 40%.Soil analysis prior to planting indicated a need for phosphate and pHadjustment. Superphosphate was applied to address the phosphate needsand the low soil pH (5.6) was amended to pH 6.5 by applying dolomiticlime. Nutrition requirements were based on annual leaf analyses andfertilizers spread under the tree by hand. There is a deficiency of thetrace elements zinc and boron which are supplemented annually. Soilerosion is prevented by planting an annual legume cover crop. Soilmoisture is monitored by means of tensionmeters and irrigation isapplied by micro-sprinklers aimed to wet 100% of soil in the drip zone.Climatic data: Long term average monthly maximum (MAXT)/minimum (MINT)temperature and monthly rainfall for Westfalia Estate, Groenkloofsection.

JAN FEB MAR APR MAY JUN JULY MAX 27.8 28.1 127.2 25.7 23.9 21.6 22.3 T(°C.) MINT 17.2 17.7 16.7 13.9 9.8 5.7 5.8 (° C.) RAIN 149.3 245.4 153.8107.1 28.5 30.9 5.0 (mm) AUG SEP OCT NOV DEC YEAR MAX 23.4 24.3 25.428.5 28.9 25.3 T(° C.) MINT 7.6 10.8 13.0 14.7 16.7 12.5 (° C.) RAIN21.8 62.9 107.5 106.5 161.3 1180 (mm)

Tree size: Four year old ungrafted ‘Merensky 2’: height 2.0 meters,width 1.8 meters.

Growth habit: Medium vigor, medium size, with a spreading growth habit,whereas the ‘Duke 7’ is more upright. The vigor of the ‘Merensky 2’alone has not been scrutinized, however the vigor of ‘Hass’ scion isgrafted onto the instant cultivar as determined by trunk circumferencemeasurements in years two through four after planting in an orchard withhigh Phytophthora cinnamoni pressure is 18.1 cm in year two, 23.6 cm inyear three, and 27.6 cm in year four.

Wood: One-year old branch: Normally green, smooth bark, havinginconspicuous lenticels.

Main stem:

Bark.—New wood green (RHS 147B), old wood greyish brown (RHS 199B) and(N199B), having a corky texture.

Young shoot(flush).—The intensity of anthocyanin coloration is medium,as compared to the absent or very weak coloration in ‘Duke 7’. The colorof the flush is reddish (RHS 176A), whereas the ‘Duke 7’ is a lightyellow-green (RHS 144B).

Lenticels.—The plant has medium conspicuousness of lenticels, whereasthe ‘Duke 7’ variety is strong. The color of the lenticels in both thepresent variety and the ‘Duke 7’ is purplish (RHS 187C). The lenticelsare 1 mm long, and have a concentration of 30 lenticels per square cm.

Foliage:

Type.—Single leaf.

Young leaf.—The present variety has an orange-brown (RHS 172A) color onits upper side, whereas the ‘Duke 7’ variety has a light orange-green(RHS 199A). The glossiness of the young leaf on flush in medium in thepresent variety. The lower surface color and texture for the undersideof new foliage is a grey-orange color (RHS 174A), dull leaves graduallyexpand and mature to a green color as the growth flush terminates.

Mature leaf.—The present variety has a length of approximately 17centimeters and a width of approximately 7 centimeters, where the ‘Duke7’ variety has a length of approximately 11.5 centimeters and a width ofapproximately 5.0 centimeters. Thus, the ratio of length to width in thepresent variety is 2.4, whereas in the ‘Duke 7’ variety it is 2.3. Theshape of the leaves of both the present variety and ‘Duke 7’ variety islanceolate to elliptic. The upper side of a mature leaf in the presentvariety and of the ‘Duke 7’ variety is of a medium gloss, having a darkgreen (RHS 147A) color. The color of the lower side of a mature leaf inthe present variety is medium green (RHS 147B), whereas in the ‘Duke 7’variety it is blue green (RHS N138B). Both the present variety and the‘Duke 7’ variety have prominent veins on the lower side of the matureleaf. The mature leaf of the present variety is generally flat in crosssection, whereas the ‘Duke 7’ is folded upwards. Whereas reflexing ofthe apex of a mature leaf is present in the ‘Duke 7’ variety, it isabsent in the present variety. The colors of the petiole of the matureleaf is generally the same as the ‘Duke 7’ variety, that is yellow-green(RHS 145A). Similar to the ‘Duke 7’ variety, the mature leaf of thepresent variety has an anise aroma. Old foliage is medium green (RHSN138C) with dull finish and fully expanded. Veins are prominent and inrelief, yellow-green in color (RHS N144A).

Fruit:

Quality.—Very good quality, whereas the ‘Duke 7’ variety can be watery.

Form.—The shape of the fruit of the present is obovoid, whereas thefruit of the ‘Duke 7’ variety is obovate.

Size.—The present variety of fruit has a length of approximately 11centimeters, and a width of approximately 7 centimeters for a length towidth ratio of 1.6. By comparison, the size of the mature fruit of the‘Duke 7’ variety is 8.9 centimeters in length by 5.5 centimeters inwidth, or the same ratio of length to width as the present variety.

Skin color.—When ripe, the fruit of the present invention has a verydark green (RHS 137A) skin color.

Skin texture.—The texture of the skin is similar to that of ‘Duke 7’variety, and very smooth.

Longitudinal ridges.—Whereas longitudinal ridges are absent in the ‘Duke7’ variety, one strong and long longitudinal ridge is present in thepresent variety.

Skin.—The skin of the present variety is very thin and membranous, andthe adherence of the skin to the flesh of the fruit is very strong. Inthe ‘Duke 7’ variety, the skin is thin, but the adherence of the skin tothe flesh of the fruit is weak.

Color.—The main color of the flesh of the fruit of the present varietyis light yellow (RHS 154D) and, with the color of the flesh next to theskin being yellow-green (RHS 144A). The width of the more intenselycolored area of the flesh next to the skin is approximately 3millimeters. Fibers in the flesh are conspicuous.

Seed.—The seed of the present variety is approximately 4.8 centimetersin length and approximately 4.2 centimeters in width. By contrast, theseed of the ‘Duke 7’ variety is approximately 4.4 centimeters in lengthand approximately 3.4 centimeters in width. The seed shape (inlongitudinal section) of both the present variety and the ‘Duke 7’variety is ovate. The color of a fresh seed coat of the present varietyis orange-brown (RHS 165A).

Bloom: Similar to the ‘Duke 7’ variety, the present variety begins toflower and bloom in August, in South Africa. The flowering time inCalifornia has not been carefully studied. Bloom period in California ofthe instant cultivar, as with other avocado varieties, varies accordingto latitude and temperatures. However, a ‘Merensky 2’ located in Irvine,Calif. has been found to bloom between November 10^(th) through February25th.

Harvest: In South Africa, the present variety is harvested in March,whereas the ‘Duke 7’ is harvested in February. Although not carefullystudied in California, it is believed that the fruit will be harvestedsometime in November to as late as May. Fruit is not used from thepresent variety. Instead, the variety is used to produce cuttings andthus propagated as a rootstock, due to its resistance to Phytophthoracinnamoni. Shoots of the present variety are grafted with another fruitbearing variety above the area where the rooting occurs.

Flower: The present cultivar belongs to Group B (female opening occursin the afternoon and male opening the next morning). The instantcultivar inflorescence is similar to that of the ‘Hass’ varietyregarding size, shape, color, anthers, pistils, petals and number, asshown in FIG. 7. The flowers of the plant do not have a distinctfragrance.

Type.—Synchronous dichogamy, and borne in panicles.

Bud size.—Approximately 6 mm in length and approximately 4 mm indiameter.

Bud shape.—Ovoid.

Bud color.—Commonly near yellow green group RHS 149D.

Opening.—The present cultivar belongs to Group ‘B’: female opening (i.e.with a mature pistil) occurs in the afternoon, the flower closes overnight, and male opening (i.e. with mature stamens) occurs the nextmorning; the flower's opening cycle lasts 20-24 hours. The ‘B’ flowertype is the compliment of ‘A’ (‘Hass’). Commonly avocados of the‘B’-type are used for enhancing pollination of ‘Hass’.

Petals.—Borne in two whorls or three perianth lobes. The petals possessentire margins and petal coloration is near yellow green RHS 145D.

Stamen.—There commonly are nine fertile stamens with each having twobasal orange nectar glands, and three stamisoda. The anthers aretetrathecal.

Pistil.—The single pistil with a slender style and small stigmaticsurface has one carpel with one ovule. The ovary is superior.

Pedicel.—Commonly approximately 7 mm in length and approximately 1.8 mmin diameter. The coloration is near yellow green RHS 145C.

Number of flowers.—There are approximately 110 to 170 flowers perinflorescence.

Full bloom.—Bloom period at Westfalia Estate Groenkloof section varieswith temperatures. However, a ‘Merensky 2’ located at the Groenkloofsection has been found to bloom from July 3^(rd) through September25^(th).

Fragrance.—The flowers of the plant are not fragrant.

Disease: Whereas the ‘Duke 7’ variety has medium resistance toPhytophthora cinnamoni, the present variety is significantly moreresistant to Phytophthora cinnamoni than the ‘Duke 7’ variety.

The tree and its fruit herein described may vary in ‘Merensky2’, asherein described and illustrated.

What is claimed is:
 1. An avocado tree as shown and described,characterized by superior resistance to root rot, and bearing more fruitwhen ‘Hass’ is grafted thereon than ‘Hass’ grown on ‘Duke 7’.